Equipment for producing a closed-section cross-member, comprising an adaptable-length punch and/or matrix, and corresponding production method

ABSTRACT

A piece of equipment for production of a closed-section cross-member configured to connect two longitudinal arms of a flexible axle of a motor vehicle. The equipment includes at least one matrix configured to cooperate with a punch to form a torsion area of a certain length on the closed section of the cross-member and a mechanism to maintain the cross-member in position. The length of the matrix and/or the punch can be adapted to adapt the length of the torsion area.

The field of the invention is that of motor vehicles. More specifically,the invention relates to non-rigid axles for motor vehicles.

It will be recalled that a non-rigid axle is the term generally used todenote an axle designed to form a torsion element between two wheels.

Conventionally, a non-rigid axle comprises two longitudinal arms eachcarrying a support for mounting a wheel, and which are connected by atransverse connecting element known as a cross-member or profile.

During axle design there are two parameters which, amongst others, areconsidered in order to assess the quality of the axle. These are bendingand torsion.

The principle of non-rigid axles allows a great deal of flexuralstiffness to be combined with a relative torsional flexibility. Ingeneral, it is through the geometry of the cross section of thecross-member, via its bending and torsional moments of inertia, that thedesired compromise between flexural stiffness and (relative) torsionalflexibility is reached.

The last few years have seen a significant evolution of non-rigid axletechniques to the lower- and mid-range sectors of the motormanufacturing market, by virtue of the numerous advantages they have tooffer, these including an excellent compromise between behavior anddesign and the fact that they are economic to produce chiefly usingassemblies of the all-welded construction.

These advantages are leading ground-contact system designersconsistently to push the technique to its absolute limits. Indeednon-rigid axles have actually run up against a certain number oflimitations including a delicate compromise between the longitudinal andtransverse stiffness and a life that is governed by the durability ofeach of their component parts which are subjected to significant elasticdeformation.

The ever increasing demand for better comfort and drivability has inparticular directed non-rigid axle design toward solutions that involveintroducing a torsional stiffness between the two trailing arms, thiscommonly being known as an “anti-roll bar” or “ARB”, with a view tolimiting vehicle body roll under cornering while at the same timemaintaining good vertical flexibility in the axle assembly as this isguaranteed to filter out the transmission to the body of irregularitiesin the road surface.

However, the expansion of non-rigid axle technology to heavier vehicles(large saloon cars, monospace vehicles or even utility vehicles) withoutin any way coming to terms with the quality of the behavior, is leadingthe most heavily stressed components ever closer to their absoluteoperating limits, whether this be static resistance to incidentalforces, or fatigue strength in respect of a loading cycle representativeof the life of the axle assembly.

The connecting element, or cross-member, therefore forms one of thecomponents that is the trickiest to develop, particularly from theendurance and behavioral standpoints.

At the present time, in non-rigid axles, the cross-member that connectsthe longitudinal arms is produced using two different technologies.

With a first technology, the cross-member is produced from a sheet metalelement that is bent (or pressed) to give it a U-shaped, V-shaped orL-shaped cross section. These cross-members generally have to becombined with an anti-roll bar to provide the axle with torsionalstiffness.

The second technology involves incorporating the anti-roll stiffnessfunction into the cross-member.

In this case, the cross-member is manufactured from a tube generally ofcircular cross section, the tube being subjected at least in its centralpart to a deformation step (at the outcome of which a portion of thewall is crushed against another portion of the wall) in order to obtainthe desired torsional and flexural stiffnesses (examples: Peugeot 806(trade name) or Opel Zafira (trade name)).

The wide variety of anti-roll stiffnesses needed to suit therequirements is provided by altering the cross-sectional shape of thecross-member and/or by changing the thickness of the tube.

The invention applies to cross-members produced using this secondtechnology, these correspondingly being known by the names of“closed-profile cross-members” or “closed-section cross-members”.

In general, all non-rigid axles that use a cross-member made up of atube have in their central region a concavity of U-shaped or V-shapedcross section.

The cross-member tube is deformed only in a transverse portion in orderto reduce its torsional stiffness and maintains cross sections of highertorsional inertia (for example circular cross sections) at the ends, tomake it easier to weld to the suspension arms.

In either one of the technologies just mentioned, the cross-member of anon-rigid axle is characterized by a high flexural stiffness and a lowtorsional stiffness, it being necessary for the latter to be gagedprecisely (associated with the anti-roll stiffness of the axle).

As far as the closed-section cross-members are concerned, the entiretyof the stiffness is afforded by the cross-member, the torsional inertiaof which has been carefully chosen to suit the anti-roll stiffnessrequired in the technical specification. As a result, the tolerance onthe anti-roll stiffness of the axle is entirely dependent on that of thecross-member itself.

During the manufacture of closed-section axle cross-members (bydeforming a tube), the manufacturer is generally faced with thedifficulty of complying with the anti-roll stiffness tolerance specifiedby the motor vehicle manufacturer through the technical specification.

One of the factors that influence the anti-roll stiffness is the spreadon tube thickness.

The problem is that of reducing the sensitivity of axle anti-rollstiffness to spread on tube thickness.

The orders of magnitude are such that a spread of +/−0.1 mm on the tubethickness may give rise to something like three times the axle anti-rollstiffness tolerance band in the technical specification.

Several solutions have been applied to this problem of keeping controlover the anti-roll stiffness of a closed-section twist-beam axle.

A first solution is to work with tube suppliers to gain control overtube thickness spread (which results from a rolling and weldingprocess), using a sorting method.

In practice though, the potential for such a solution is relativelylimited.

A second solution is to use tubes from a rolling and welding processwhich are then re-drawn in order to improve the tolerance on thethickness.

This solution proves effective but the associated cost is high. Ittherefore increases the overall cost of the axle, this being somethingwhich is usually incompatible with the demands of motor vehiclemanufacturers.

According to another solution, recourse is had to a hot pressing processwhich provides better control over the geometry of the sections formed.

However, it has been found in practice that this technique is unable toreduce the sensitivity to spread on the thickness.

According to yet another solution, the sheet metal of the tube, at eachend of the cross-member, is connected using a method of “clinching” (inwhich one metal sheet is driven into the other by punching).

This solution provides better control over the spread on the actuallength of the working section (the section with the lowest torsionalinertia), but does not in any way compensate for spread on the tubethickness.

It is a particular objective of the invention to alleviate thedisadvantages of the prior art.

More specifically, it is an objective of the invention to propose atechnique for the manufacture of a closed-section cross-member for anon-rigid axle which allows better control over the spread on thetorsional stiffness of the cross-member by comparison with the solutionsof the prior art.

Another objective of the invention is to provide a technique such asthis which is simple in design and easy to implement.

Another objective of the invention is to propose a method of manufacturecorresponding to such a technique.

These objectives, together with others which will become apparent lateron, are achieved by virtue of the invention the subject of which istooling for manufacturing a closed-section cross-member intended toconnect two longitudinal arms of a motor vehicle non-rigid axle, saidtooling comprising at least one die intended to collaborate with a punchto form, on said closed section of said cross-member, a length oftorsion zone, and means for holding said cross-member in position,characterized in that said die and/or said punch are length-adjustableso that said length of said torsion zone can be adapted.

This then yields an adaptive system allowing the length of the torsionzone (also known as the “working zone”) of the cross-member, andtherefore its torsional stiffness, to be varied.

Such variation in the length of the torsion zone can be obtained simplyby adjusting the dimensions of the die and of the punch of the shapingtooling, and to do so in a way integrated into the tooling as willbecome more clearly apparent later.

According to a preferred solution, said die and said punch each compriseat least two parts that can be moved away from/toward each other.

This then yields a particularly simple and effective way to adapt thetooling to suit the desired torsional stiffness of the cross-member.

According to a first embodiment, said two parts of said die and/or ofsaid punch can be actuated by at least one actuating cylinder.

According to a second embodiment, said two parts of said die and/or ofsaid punch are held in the continuation of one another using screwingmeans.

In this case, the tooling preferably comprises a set of shims which canbe interposed between said two parts of said die and/or of said punch.

The set of shims may then comprise a variety of shims of differentthicknesses capable of covering a given range with the desiredprecision.

According to an advantageous solution, said holding means comprise atleast one variable-travel clamp.

Clamps such as this contribute to the modular nature of the tooling,allowing it to be adapted to suit both the thickness of the cross-memberand the length thereof.

The invention also relates to a method for manufacturing aclosed-section cross-member intended to connect two longitudinal arms ofa motor vehicle non-rigid axle, using tooling comprising at least onedie intended to collaborate with a punch to form, on said closed sectionof said cross-member, a length of torsion zone, and means for holdingsaid cross-member in position, characterized in that it comprises a stepof adjusting the length of said die and/or of said punch so as to adaptsaid length of said torsion zone.

As a preference, the method comprises a prior step of calculating saidlength of said torsion zone according to the desired torsional stiffnessof said cross-member and according to the wall thickness of saidsection.

According to an advantageous solution, the method comprises a step ofadjusting the travel of two clamps that form said holding means.

Other features and advantages of the invention will become more clearlyapparent from reading the following description of a preferredembodiment of the invention which is given by way of entirelynonlimiting illustrative embodiment and from the attached drawings amongwhich:

FIGS. 1 to 3 are each a view of one step in the manufacture of aclosed-section cross-member according to the prior art;

FIGS. 4 a, 4 b, 5 and 6 are views illustrating the influence of thetorsion zone of a cross-member on the torsional stiffness thereof;

FIG. 7 is a schematic view of the overall principle of the invention;

FIG. 8 is a schematic view of a preferred embodiment of the invention;

FIGS. 9 to 11 are graphs illustrating one example of a tolerance band onthe torsional stiffness that is obtained by virtue of the invention, asa function of a given spread on thickness and a given variation onworking length.

With reference to FIGS. 1 to 3, it will be recalled that aclosed-section cross-member is manufactured according to the prior artfrom a tube 1 which has its cross section deformed using a press inorder to produce a torsion zone.

The conventional tooling comprises a punch 2 and a die 3 which areactuated by a press and are intended to collaborate with one another toform the torsion zone, and clamps 4 intended to press against the endsof the tube 1.

In an initial phase, the tube is placed in the press, and the clamps arebrought up against the tube in order to immobilize it, then the press isclosed.

FIG. 2 illustrates the step of forming the torsion zone on a given andfixed length, the die 3 and the punch 2 crushing the tube between them.

At the end of the forming step, the press is opened (the die 3 and thepunch 2 therefore being moved away from each other) and the clamps areretracted from the tube (FIG. 3).

With reference to FIGS. 4 a and 4 b, it will be recalled that thetorsion zone of a closed-section cross-member is the name given to thezone L corresponding to the central part of the cross-member.

For such a cross-member, the geometric profile of the cross section ofthe tube, the tube thickness and the length of the torsion zone are keyfactors in obtaining the torsional stiffness of the cross-member.

In addition, for a given section profile and tube thickness, thevariation in the length ΔL of the torsion zone varies the torsionalstiffness R of the cross-member in the following way: if L increases, Pdecreases, and vice versa.

Thus, when the length of the torsion zone L is minimal, the cross-memberhas maximum torsional stiffness (FIG. 5). Conversely, when the length ofthe torsion zone L is maximal, the cross-member has minimal torsionalstiffness (FIG. 6).

As mentioned previously, the principle of the invention illustrated byFIG. 7 lies in making the punch 2 and the die 3 length-adjustable sothat they will allow variations in length ΔL corresponding to the lengthof the torsion zone required.

In order to do this, according to the embodiment illustrated by FIG. 8,the punch 2 and the die 3 each have two parts, 2 a, 2 b and 3 a, 3 b,respectively, that can be moved towards or away from one another inorder to vary ΔL.

The separation between the parts 2 a, 2 b on the one hand, and the parts3 a, 3 b on the other hand, is obtained by interposing one or more shims6 between them.

Of course, the number and the thickness of the shims are chosen to suitthe desired ΔL.

The parts 2 a, 2 b of the punch are held together with the shims 6 usingthreaded rods 5. The same is true of the parts 3 a, 3 b of the die andthe shims 6.

Furthermore, the travel of the clamps 4 is controlled by hydrauliccylinder actuators (not depicted) that perform the translationalmovement of the clamps.

It will be noted that the lengths of the punch and of the die can bealtered hydraulically, for example using on-board cylinder actuators,according to another conceivable embodiment.

The adaptive adjustment of the tooling which has just been described isperformed between each production run (a run being defined by a batch oftubes that are characterized by a particular mean tube thickness), onthe press, with or without partial disassembly of the tooling.

Prior to setting up the tooling, the length of the torsion zone iscalculated according to the tube thickness, this being for a stiffnesslaid down in the technical specification.

Such tooling therefore, in production terms allows components to beformed in runs.

FIGS. 9 to 11 are graphs illustrating an example of a torsionalstiffness tolerance band obtained by virtue of the invention as afunction of a given spread on thickness and a given variation in workinglength.

The graph of FIG. 9 indicates a spread (of between 3.35 mm and 3.55 mm)in tube thickness across a batch of tubes.

The graph of FIG. 10 indicates variations in working length(corresponding to the length of the torsion zone).

The graph of FIG. 11 indicates the output data relating to the torsionalstiffness.

In the above example, the variation in working length (denoted L in FIG.4 a) between 618 and 700 mm makes it possible to comply with a stiffnesstolerance band of ±2 m.daN/° for a tube thickness tolerance of ±0.1 mm.

Without altering the length of the working zone (that is to say usingtechniques of the prior art) the spread on the tube thickness leads to aspread of ±7.25 m.daN/° (±8%) on torsional stiffness.

1-9. (canceled)
 10. A tooling for manufacturing a closed-sectioncross-member configured to connect two longitudinal arms of a motorvehicle non-rigid axle, the tooling comprising: at least one dieconfigured to collaborate with a punch to form, on a closed section ofthe cross-member, a length of torsion zone; and means for holding thecross-member in position, wherein the die and/or the punch arelength-adjustable so that the length of the torsion zone can be adapted.11. The tooling for manufacturing a closed-section cross-member asclaimed in claim 10, wherein the die and the punch each comprise atleast two parts that can be moved away from/toward each other.
 12. Thetooling for manufacturing a closed-section cross-member as claimed inclaim 11, wherein the two parts of the die and/or of the punch can beactuated by at least one actuating cylinder.
 13. The tooling formanufacturing a closed-section cross-member as claimed in claim 11,wherein the two parts of the die and/or of the punch are held incontinuation of one another using a screwing mechanism.
 14. The toolingfor manufacturing a closed-section cross-member as claimed in claim 13,further comprising a set of shims configured to be interposed betweenthe two parts of the die and/or of the punch.
 15. The tooling formanufacturing a closed-section cross-member as claimed in claim 10,wherein the means for holding comprises at least one variable-travelclamp.
 16. The tooling for manufacturing a closed-section cross-memberas claimed in claim 11, wherein the means for holding comprises at leastone variable-travel clamp.
 17. The tooling for manufacturing aclosed-section cross-member as claimed in claim 12, wherein the meansfor holding comprises at least one variable-travel clamp.
 18. Thetooling for manufacturing a closed-section cross-member as claimed inclaim 13, wherein the means for holding comprises at least onevariable-travel clamp.
 19. The tooling for manufacturing aclosed-section cross-member as claimed in claim 14, wherein the meansfor holding comprises at least one variable-travel clamp.
 20. A methodfor manufacturing a closed-section cross-member configured to connecttwo longitudinal arms of a motor vehicle non-rigid axle, using toolingincluding at least one die configured to collaborate with a punch toform, on a closed section of the cross-member, a length of torsion zone,and means for holding the cross-member in position, the methodcomprising: adjusting a length of the die and/or of the punch so as toadapt the length of the torsion zone.
 21. The method of manufacturing aclosed-section cross-member as claimed in claim 20, further comprising aprior calculating the length of the torsion zone according to a desiredtorsional stiffness of the cross-member and according to a wallthickness of the section.
 22. The method of manufacturing aclosed-section cross-member as claimed in claim 20, further comprisingadjusting travel of two clamps that form the means for holding.
 23. Themethod of manufacturing a closed-section cross-member as claimed inclaim 21, further comprising adjusting travel of two clamps that formthe means for holding.